Overcharge is a dangerous abuse of lithium-ion batteries. Overcharge generally occurs when a current is forced through the batteries and the charge delivered exceeds the charge-storing capability of the battery. Overcharge of lithium-ion batteries can lead to the chemical and electrochemical reaction of battery components, rapid temperature elevation, and can also trigger self-accelerating reactions in the batteries and even explosion. A redox shuttle is a chemical compound that is incorporated as an overcharge protection mechanism for lithium-ion batteries. Generally, the redox shuttle can be reversibly electrochemically oxidized and reduced at a potential slightly higher than the working potential of the positive electrode of lithium-ion batteries. With the incorporation of a redox shuttle into the electrolyte, the lithium-ion batteries can normally operate in a voltage range under the redox potential of the redox shuttle. If the battery is overcharged, the battery voltage will meet the redox potential of the redox shuttle first and activate the redox mechanism of the redox shuttle, which will proceed as the only active component to transfer the excessive charge through the battery without causing any damage. Under such a mechanism, the dangerous voltage of the battery will never be reached even when being overcharge-abused.
The research and development of redox shuttles for lithium-ion batteries can be traced back to 1980s. However, only two classes of redox shuttles are known. The initial effort was focused on derivatives of ferrocene, which are suitable for the 3 V class of lithium ion-batteries. It has been reported that certain aromatic compounds can be redox shuttles for state-of-the-art lithium-ion battery technology. However, the quantitative structure-activity relationship of the aromatic compounds is not well understood. For example, Adachi has proposed fluorinated dimethoxybenzene as a promising redox shuttle for 4 V class lithium-ion batteries in U.S. Pat. No. 5,763,119. However, it was later shown that none of the claimed fluorodimethoxybenzenes is stable enough to survive the basic low current overcharge test. J. Chen, C. Buhrmesters, and J. R. Dahn, Electrochem. Solid-State Lee., 8(1): A59-A62 (2005). The only structure proved to sustain overcharging for thousands of hours is 2,5-di-(tert-butyl)-1,4-dimethoxybenzene reported by Chen and coworkers. A drawback of this compound is that its redox potential is about 3.9 V vs. Li0, and can only work for LiFePO4 positive electrode materials. Therefore, the compound cannot be used with the widely used, and commercially available positive electrode materials such as LiMO2 (M=Co, Ni, Mn). The LiMO2 (M=Co, Ni, Mn) materials have working potentials up to 4.2 V vs. Li0. Therefore, redox shuttle additives with a range of redox potentials, but especially those at 4.4-4.5 V vs. Li0, are highly desired for universal application in current lithium-ion battery technology.